tape head amp issues

[jatalahd]

The engage/release mechanism looks a bit complicated, and suitable springs might be hard to find in different parts of the globe if you are planning to release this as a DIY project. I would suggest using a simple screw mechanism, which would make the design more versatile considering different head heights and optimal head mounting depth with respect to the cassette/tape. As the tape heads also have different proportions all around, all heads might not align properly in the middle of the tape, so some fine tuning option should be considered there.

Also I would suggest using separate mounting mechanism for all heads, or at least a fine tuning option for each. For optimal performance you could consider how to include proper adjustment for the azimuth on that engage plunger.

[AdamB]

From the drawing, it looks like a 45deg angle between the movement faces. As only the head needs to push the release slider across and not the other way around, a shallower angle in favour of the head slide could be better?

Yea that's a good idea, I'll play with that. So far, I've found its only the release plunger that jams, and only on the way out (the spring doesn't have enough force to overcome the friction with the bottom and top layers it's sandwiched between). I've made the plunger a tad thinner to give it some breathing room and also sanding it a little (as I'm not currently post-processing my prints at all) will probably help, to give it a smooth surface. And the lubricant will also help hopefully.

I would suggest using a simple screw mechanism

That's an idea - something I'll think about. My issue with screw mechanisms is that they are a pain to print with. If you print a screw mechanism that uses metal screws but also needs to be adjusted/re-used, then you have to use threaded inserts which need a good deal of plastic around them to make them stable. This increases the size of the part quite a bit around where the insert fits. If you don't use inserts the screws just strip the thread in the plastic very quickly. I think it may also be the case that the screws are only fully secure when they reach the end of their travel. Alternatively, if we printed a screw in plastic, they tend to need to be quite big and suffer from wear quite a lot - I'd probably have to switch to something slippery and tough (but hard to print with) like nylon.

I'd also worry about how quickly you could undo the screws/re-fasten them to switch tapes - seems like it wouldn't be something you could do between songs at a gig.

For finding springs - we might be able to 3D print a "spring" mechanism - similar to how snap-to-close enclosures work; just a thin U shaped sprung bit of plastic. One of the tape transports I took apart used this type of spring on a release plunger.

I am a little concerned about other people being able to just print it off and mount their parts on (heads especially) - as you say heads have different mountings and dimensions. I've seen people release designs where the user can fill in some values to customize the print before they do it, so maybe we can do this (I think it's called parametric modelling). But yea I've no idea how to do that yet - I'll figure that out once my own one works

I would like to add some way to adjust the head azimuth, but I think for the first version we just need to get it working, then we can start getting fancy once v1 is done. I've no experience with CAD so it's quite hard going designing mechanisms that will print and go together properly! I'd rather keep it simple for now - perfection is the enemy of progress.

[anotherjim]

Sometimes I see sliding parts where they make most of it too narrow and have a few lobes sticking out that keep it centred. Presumably, that reduces sliding friction a lot.

[AdamB]

Sometimes I see sliding parts where they make most of it too narrow and have a few lobes sticking out that keep it centred. Presumably, that reduces sliding friction a lot.

Yea I've definitely seen that approach in other transports I've disassembled. The problem is making it printable - coz to reduce friction as much as possible you'd want the lobes sticking out top and bottom, which when you print would mean that the bulk of the print is off of the print bed (an overhang), which will either fail to print (coz the extruder is printing into the air - there's nothing for the plastic to bond to) or you need to add supports to the print which I'm trying to avoid. But definitely room for improvement in there - I could make at least 1 side have less friction by keeping the base of the plunger flat and then putting little lobes on the top surface.

[anotherjim]

You do have two planes to work in. Bumps on the wide horizontal sliding surfaces and nodes on the narrow vertical surfaces. An advantage is you can make it with tight tolerances and correct any tightness with a flick of a file.

Another latch type comes to mind. The head slide can have a built-in springy finger. When it slides in, the fingertip springs into a recess and locks it. The release pushes the fingertip out of the recess. The release doesn't actually need a spring, just preventing it from falling out. The head slide still needs a spring.

[RLawlor]

Hello, I've decided to try build my own tape echo also. I've used the EP3 preamp and playback circuit and have tried the bias oscillator provided by jatalahd, however I can't seem to get it to work. I've tested all parts of my circuit, it seems the tape playback, preamp and recorder amplifier all work but as soon as I turn on the bias oscillator there is a very loud noise that drowns out everything else. I'm regulating down 24V to power the oscillator and recorder amplifier and it seems to be getting hot even with a heat sink, as are the transistors in the bias oscillator.

I directly copied the breadboard layout so I'm not entirely sure what's gone wrong  ! It seems that the oscillator is working as I'm getting a decently high voltage at the output but the noise is unbearable, if anyone has any ideas it'd be much appreciated!

Ronan.

[jatalahd]

If anyone has any ideas it'd be much appreciated!

In my first breadboard prototype builds, I had some problems with loud motorboating low-frequency oscillation, but the reason was not the bias oscillator, only the high-gain playback amplifier was going crazy. But in EP3 playback amplifier the gain is quite modest, so cannot directly see that as a problem. For me the problem was eventually in the power source filtering and physical connections (jump wire lengths, distance between wires) in the breadboard. After soldering the build to veroboard, I have not had any problems, but I have only used battery power in my build. I would assume a mains-connected PSU would cause noise problems.

Without more detailed information about the problem, I can just give some general advice to isolate it (assuming that the noise is heard in the output of the playback amplifier):

- enhance the psu noise filtering in the playback amplifier to cut out lower frequencies (bigger filtering cap)
- add a 3.3 nF cap at the base of Q1 in the playback amplifier, to see if any difference in noise
- use a star grounding scheme, and connect Vdc+ further away from the ground connection
- keep bias oscillator components and wires far away from rest of the circuit
- test the circuit without the playback head connected, if noise goes away, then the head is picking up the noise from ground. Try the same also with the record head.

Also, as you noticed, running the bias oscillator with 24V is not a good idea. I would suggest 12V maximum.

[AdamB]

Quick update on where I'm at.

I found that driving the tape with just the take up reel didn't really work. There was too much friction with the 3 heads for the motor to pull the tape through unless the motor was at a high RPM - which meant the useable delay times would be quite limited to short times - and it just wasn't very reliable.

So, for now I've ditched the cassette. The problem is that as Jatalahd suggested, I need a pinch roller to tension the tape, give traction and pull the tape through the heads - but I can't do that with a cassette and have an erase head due to lack of room.

So this is the latest prototype (which still doesn't work lol):
https://www.dropbox.com/s/mhzjl34t01o24n8/PXL_20210330_082838824.jpg?raw=1

The metal shaft (top left) that presses against the roller is powered, as well as the reel. This is achieved using a double-pulley system. I will at some point probably need to play around with different pulley size ratios (I imagine the reel needs to run slightly faster than the roller in order to tension the tape) but for now I don't need it to be perfect, I just need it to work!

Current issues are;
-The tape doesn't run against all 3 heads. I think I need to go with a pad or roller between the head and the tape.
-Tape alignment to the heads is off, I need to tweak the mountings a little.
-The pinch roller works for a short while, but then the tape slips up or down the roller until it isn't between the roller and the metal shaft anymore. I suppose I need some sort of guide to keep it in the right place.

But no matter, we'll figure it out!

[jatalahd]

I need a pinch roller to tension the tape, give traction and pull the tape through the heads - but I can't do that with a cassette and have an erase head due to lack of room.

If you use a combined erase&record head, then you would have space for pinch roller in the cassette. I have seen those combined heads available in AliExpress (have not ordered them though, so cannot know are they actually usable). Also even when using the cassette, since you are building a custom made machine, you could try to place the pinch roller to the side of the cassette and make a hole at the side of the cassette to take the tape out so that it travels around the pinch roller mechanism. Then you would have room for 3 heads... Or the erase head could be placed to the side of the cassette, leaving room for the pinch roller at the "normal" position.

-The pinch roller works for a short while, but then the tape slips up or down the roller until it isn't between the roller and the metal shaft anymore. I suppose I need some sort of guide to keep it in the right place.

This might turn out to be tricky to get it working properly. I had this effect using the "official" tape player mechanism, in my case it was related to head misalignment, which in some cases took the tape slightly out of the center line of the tape path and eventually caused the tape slipping out under the pinch roller.

[BeliaEllet]

Hi...I accept you've cleaned the head (with a cotton bud and propan-2-ol, not a cleaning tape). If not do as such.

Have you demagnetised the head? I once had a reel-to-reel video recorder that gave a low abundancy signal from the tape that imrproved terrifically when I did that.

[AdamB]

If you use a combined erase&record head, then you would have space for pinch roller in the cassette.

Thanks for the tip, that certainly sounds like it needs investigating! I'll see if I can get hold of one. I have switched design to a non-cassette version, but I have all the old revisions of my designs saved so I can switch back to cassettes very easily. I'll see what I can find, coz I'd definitely prefer the cassette style design.

So here's where I'm currently at with the transport:

I need to try it out with the variable speed control from the Arduino, going to hook that up again tonight and see how it fairs. So far putting the little doodad over the roller at the top left seems to be enough to stop the tape wandering off the roller (I think it was actually the roller itself travelling up the shaft, so now it's held nicely in place).

Also needs some way to keep the reel on the spindle, so far gravity has been doing that job for me. Will think something up.

[RLawlor]

variable speed control from the Arduino

I used a 555 PWM circuit which worked quite well. Good thing about it is that you can change the PWM by just changing a cap, although if you wanted anything fancy with the motor speed the Arduino would work better! I found that a low frequency tended to improve the torque of the motor and helped move the tape more smoothly through my mechanism.

[anotherjim]

If you abandon the cassette you have something like Roland Space Echo where the capstan/pinch-roller does all the tape transport and felt pads keep head contact. The endless tape itself just bunches in a box area with a transparent cover.

I think the best tape transport I've seen was the old Data cartridge introduced in the 70's. The outside of both spools was driven by a continuous band and the tension keeps constant as one spool increases in diameter, the other decreases. The band is driven by a hub in contact with a rubber drive "puck" on the motor. The recording side of the tape only contacts the heads.

[RLawlor]

I think I've got everything working, however the bias oscillator only gives 50VPP when unloaded and as soon as I connect the record amplifier it drops to around 8VPP. Anyone have any ideas on how to stop the oscillator getting loaded down so much? Any help would be much appreciated!

[anotherjim]

Maybe insert an emitter follower NPN buffer before R33 & R34?

[jatalahd]

I think I've got everything working, however the bias oscillator only gives 50VPP when unloaded and as soon as I connect the record amplifier it drops to around 8VPP. Anyone have any ideas on how to stop the oscillator getting loaded down so much? Any help would be much appreciated!

Yes, I had made more improvements on my prototype schematic regarding this. The answer is to use the bias trap between the oscillator and the recording amplifier. Here is my latest revision schematic, notice the bias trap on the middle of the right side:



With those values it is not automatically tuned correctly, but will improve the situation nicely. If possible try to tune it to maximum oscillator signal at the tape head. In my own proto, I used two parallel caps, 390n + 330n to make a 720n cap in parallel with the 10 mH inductor, which gave me a maximum of 44 V rms bias voltage (about 100 vpp) at the record head even when loaded with the recording amplifier. Note that the inductor should be around 10 mH, preferably more to give better notch filtering. I simulated using different sized inductors and came to the conclusion that 10 mH to 33 mH inductor would be optimal.

By the way, the unloaded bias oscillator voltage of 50 vpp is quite small compared to what I am getting. If the quality factor if the inductor in the bias oscillator is poor (too much series resistance compared to the inductance, that is QL = L/RL) it will reduce the amplitude. Also, the ratio of the capacitors in the oscillator is important. If it is something else than 10n, 200n, 1u, then it will affect the output amplitude for sure. It is a bit tricky unfortunately. Also measuring with low-impedance probes will load the oscillator and show a lower reading on the voltage.

Hopefully this helps.

[RLawlor]

Thank you very much for the help! I substituted the inductor for the erase head in my circuit and as such replaced the 10nF capacitor with a 22nF one. Maybe I'd be better off driving one half of the record head and the erase head rather than both sides of the record head so I can use a 'better' inductor in the oscillator. As for the bias trap I thought about that but didn't realise a high value inductor was preferable as I'm only using 330uH currently. I'll be sure to try something bigger and see what I can do!

[AdamB]

OK. So. Hooking up my transport to the arduino speed controller worked fine - I had to re-print the pulleys at a slightly bigger ratio to get the torque needed for really low speeds but it's working really well now and is really stable.

Playback from the transport works fine (I cut the tape from an old cassette to make the loop and it happily plays back what was on the loop already) - however still having some trouble with recording.

So, here's what my hobbyist scope recons is coming off of the bias osc:

This one is with the erase head in place of L2 (note that if I turn down R32 to lower the PP voltage it forms more of a sine shape, so the weirdness might be it going out of the 20Vpp range of the scope)
https://www.dropbox.com/s/zvbpjjxi1bb9z0p/Scopey1.jpg?raw=1

And this one is with a 820uH inductor for L2
https://www.dropbox.com/s/gv6kzisx4crg6vq/Scopey2.jpg?raw=1

I tested my recording preamp (by just connecting an audio jack to the output) and it passes signal. Hooking the oscillator and recording amp together I don't get anything recording onto the tape. I think the erase is working, coz it's nuked what was already on the tape.

I do hear something from the oscillator through the output - kinda a low pitched 'farting' sorta noise which goes away when I either turn R32 ( to lower the PP voltage of the osc) or disconnect the osc completely.

Where do I go from here? I assume either:
-There's a contact problem with the tape on the heads (though the tape is travelling through the heads guides and is pulled reasonably firmly against the heads)
-The recording amp is not providing enough gain
-Oscillator isn't tuned right
-Oscillator signal is just nuking everything (bias trap needed)

One thing with all this; if the oscillator isn't tuned just right, will it just not work at all? Or should it be "working", just at low quality? It's hard to know how to debug this thing!

-Adam

[AdamB]

Turns out that if I turn the gain on the interface right up that you can actually make out a really faint, very distorted echo. Check out this video

There's a huge amount of noise - some of it might be from the Arduino, but I suspect not all of it.

-Adam

[jatalahd]

Nice progress!

Based on your oscilloscope screenshots, your scope is good enough to measure the output of the bias oscillator. That is good and will help the debugging a lot. The second trace with the "820 uH" inductor is quite close what it supposed to look, but there are a few issues there still.

   1. There is DC offset in the measured signal
   2. The Vrms value is a bit too low
   3. With 820 uH inductor you should get about 56 or 57 kHz, now it shows 47 kHz. With 1 mH inductor you would expect to get 50 kHz.

So as for the oscillator, you should be able to at least get rid of the DC offset. And at this point it would be good to measure the circuit so that the recording amp and the oscillator are both connected to the record head and measure the bias oscillator signal at the head. You need to have DC blocking capacitors on the output of the record amp and at the output of the bias oscillator.

Secondly you need to aim for 16 - 20 Vrms bias oscillator signal (clean sine wave), measured at the record head when both recording amp and the bias oscillator are connected to it. Bias trap will help here. If you have some old tape recorders around, you could try to find a suitable tunable bias trap inductor from those, then you could also snatch a suitable capacitor also. You need to match the 1/(2*pi)*sqrt(1/LC) of the bias trap to the frequency of the bias oscillator and with the scope, tune the bias trap so that you will get maximum Vrms bias signal at the tape head.  After this you can adjust the recording amp volume to suit your needs.

Also, if possible, I would test the recording on a "traditional" tape transport with a C-cassette first. Just connect the recording amp and the oscillator to the existing head in the transport. Then you can play it back on another cassette player to verify that you get a good enough recording.

To me it it looks like that your transport mechanism is spinning the tape too fast and quite often the tape wobbles off the surface of the tape head. This too will affect the quality of the recording, because the record head most likely cannot get proper contact to the tape to bias it properly. So I would not use that for preliminary tests of the recording circuit. The buzzing noise that you are hearing probably mostly comes from the DC motor, I have that problem too that the motor induces some noise especially when it runs fast, but not in that magnitude that it would be disturbing.

So as a conclusion, my advice would be to focus on getting a clean recording on a normal tape mechanism to finish the electronics part and after that focus on testing the transport part. I know that it might be frustrating to just try out different component values to optimize the bias oscillator, but try to snatch some components from existing tape players, so that you would not need to buy them as new. As I mentioned before, your oscilloscope will be a good help in troubleshooting.